lec 1 : introduction, conservation of energy, dimensions and units, systems, properties

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Lec 1: Introduction, conservation of energy, dimensions and units, systems, properties

Mt St Helens, 1980

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• ENGR 212-503• For next time:

– Read: §1-1 to 1-10 – HW 1 --- book not available; postponed to September

8, 2003

• Outline:– Introduction– What is thermodynamics?– New concepts

• Important points:– Systems (open/closed)– Properties (intensive/extensive)– Units

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Laws of thermodynamics

• First law of thermodynamics is same as law of conservation of energy--deals with quantities of energy

•Energy can change forms: from potential energy to kinetic energy, for example--but the total amount never changes.

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Laws of thermodynamics

• Where a system such as a human body may undergo an energy change, the first law is written:

EEE outin

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Units

• Two systems will be used heavily:

– The International System (known as the SI system from its French title--see text).

– The US Customary System (USCS). (The USCS is also known as the inch-pound (IP) system and the English Engineering System).

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Newton’s 2nd Law

constmaF

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SI system constant

• In the simpler SI system the units are chosen so that the constant is unity. The primary dimensions are mass (kg), length (m), and time (s or sec).

• The equation that contains four units is not over-determined because force F is then a derived or defined unit.

constmaF

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Force is a defined unit in SI

1 N 1 kg 1 m / s2

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USCS

• This unfortunate system has four applicable primary dimensions for the equation which has only four dimensions.

– Mass is lbm

– length is ft– time is s (sec)

– force is lbf

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USCS

• The system is over-determined

• We get around this by introducing gc, the gravitational constant, so that

cg

maF

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Gravitational constant

• In other words, the const in F=maconst is:

cg

1const

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Gravitational constant

2f

mc seclb

ftlb174.32g

•Please memorize this.

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• Your text gets around the problem of over-determination by defining:

Gravitational constant

2m

f sec

ftlb174.32lb1

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Advice which will gain you points

• 1. Write out all units in all equations to the bitterest of detail.

• 2. Use a horizontal line in all equations, not a slash.

e.g. Write ; Not F = ma/gc

cg

maF

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To gain you points

• Remember that a goal is to communicate that you know how to do the assignments--including obtaining the correct answer.

• Keep steps in order--work from top to bottom on the page--do not jump erratically to left over open space for key steps, which may then be out of order. Circle or box key numbers and results.

• Make it easy for your reader, not hard.

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Notes on expressing units

• The º symbol is not used with absolute temperatures K or R.

• Unit names are not capitalized even if they are proper names (for example, 1,000 watts).

• Abbreviations of units are capitalized if they are from proper names (1,000 W or 1.2 kWh).

• Abbreviations are never plural (4 joules but 4 J; 4 kilojoules but 4 kJ).

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TEAMPLAYTEAMPLAY

WHEN A TEAM PLAY EXERCISE APPEARS, IT IS TO BE SOLVED FIRST BY YOU, THEN BY A PARTNER, AND FINALLY AS A GROUP OF FOUR.

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TEAMPLAYTEAMPLAY

In the old movie Armageddon, actor Bruce Willis flew to an asteroid:

• What was the force on Bruce Willis, whom we shall assume weighs perhaps 200 lbm, when he stepped on the asteroid where the acceleration was an assumed 1.8 ft/s2?

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System

• A system is a quantity of matter or region of space we want to study.

• Two types of systems (closed and open).

• The first is known as a closed system (or control mass)--because it involves a fixed amount of matter and is closed to matter (mass) transfer across the system boundary.

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System

•Closed system--no mass crosses boundary indicated by dashed line.

•Mass indicated by gray is system to study.

•Boundary can move (piston could go in and out)

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System

• A system is a quantity of matter or region of space we want to study.

• The second is known as an open system (or control volume)--because it involves a fixed region in space and is open to matter (mass) transfer across the system boundary.

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•Open system--boundary generally does not move (it could) and mass crosses the boundary.

•We are just interested in the region bounded by the dashed lines.

Fluid velocity=V0

System

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System boundaries are often defined by dashed lines, as in the previous pipe flow example. The dashed lines define a control surface.

System

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TEAMPLAYTEAMPLAY

A bottle of Coke is placed in a refrigerator that is at 3C:

•Should the bottle of Coke be treated as a closed or open system?

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Definitions

• Property--A characteristic of a system to which numerical values can be assigned to describe the system.

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Definitions

• State--Condition of a system defined by its properties.

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TEAMPLAYTEAMPLAY

• What are some properties that define the liquid in the previous Coke example of a system?

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Definitions

• Extensive properties--properties which can be counted, and for which their value for the whole system is the sum of the value for subsystems into which the overall system might be divided.

• They depend on the extent of the system.

• Often symbolized with capital letters.

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Definitions

• Intensive properties are independent of the size (mass or volume) of the system.

• Often symbolized by lower case letters.

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Energy

• Energy = sum of internal energy, kinetic energy, and potential energy.

• (Note energy E here is an extensive property).

mgz2Vm

UPEKEUE2

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Energy

• On a unit mass basis, or intensive basis

• Every term in the previous equation has been divided through by the mass m.

gz2

Vupekeue2

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Other units

• Volume V will be in m3 and ft3

• Density is in kg/ m3 or lbm/ ft3

• Specific volume v is new and powerful; the inverse of density, it is in units of ft3/lbm or m3/kg.

ρ

1v

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Specific gravity

• Also known as relative density--it is the ratio of the density of a substance to some reference density.

• Reference density is usually water at 4 ºC.

• Sometimes, for gases, “gas gravity” is used, and in that case the reference density is for air at 60 ºF and 14.696 psia.

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TEAMPLAYTEAMPLAY

Sketch possible system boundaries for the following and classify them as open or closed:

• Bicycle tire deflating.• Kettle of water heating (but not boiling).• Kettle of water boiling.• Jet engine in flight.• Pentium IV, 2.4 Ghz chip in operation.